This invention relates to pads removably secured in resilient work-gripping jaws of a work-holding collet. The pads may be used in any suitable conventional collet adapted for use in metal working machines, including automatic or hand-operated screw machines, and the like, which are constructed especially for the purpose of feeding the work. More particularly, this invention relates to longitudinally sectioned collet pads consisting of at least two pad segments, which when removably secured in the jaws of the collet, coact to grip the work or stock extending through the collet, as is well-known in the art, as, for example, in an automatic screw machine. The particular subject matter which I regard as my invention is an improvement in the aforementioned collet pad segments such as are conventionally used, which improvement comprises a novel circumferentially split composite pad structure embodying a novel, yet defiantly simple concept.
The novel collet pad of my invention comprises at least two longitudinally sectioned composite pad segments each of which includes a removable and replaceable inner work-gripping liner snugly fitted to an outer member. The inner work-gripping liner may also be referred to as the inner pad member, or simply as the liner, and, the outer member as the outer pad member; since collet pads are generally cylindrical and are adapted to grip an elongated workpiece or bar of stock, the usual relationship of the inner liner to the outer member is as if the composite pad segments of my invention were circumferentially split, and the instant collet pad comprising at least two composite pad segments is referred to as a circumferentially split collet pad. As mentioned hereinabove, this circumferentially split collet pad is also longitudinally sectioned so that the composite pad segments of this invention are particularly designed for use in a radially expansible collet such as is disclosed in U.S. Pat. Nos. 1,573,048; 1,937,236; 2,689,740; 2,747,881; 2,951,707, inter alia.
Persons skilled in the art know, and will recognize from the foregoing references, that, over the past half a century or more, a great deal of effort has been expended towards improving the performance of both the collets and the collet pads used therein. These persons will also acknowledge the considerable and ever-increasing cost of fabricating conventional collet pads, and over the many years, have become accustomed to high-cost collet pads and enured to fabricating collet pads in the time-honored manner, as disclosed in the foregoing references, and summarized hereinbelow.
Conventionally, a billet of tool steel is machined to provide a desired outer configuration, including, for example, a circumferential rib intermediate the ends of that portion of the billet to be utilized as the pad. The rib is designed for engagement within a recess in the jaws of the collet. This integral pad is then provided with an axial bore of predetermined diameter, which is subsequently machined with highly specialized machine tools. Teeth are cut into the inner surface of the billet by cutting a multiplicity of circumferential grooves of predetermined depth and thereafter progressively slotting or broaching with a series of broaching tools of increasing diameter until the desired dimensions are obtained. Broaching typically provides the inner surface of the pad with serrations, teeth, or the like, which dig into the surface of the stock and grip it so tightly and securely that the stock may be subjected to a high-speed machining operation without budging. The integral pad is thereafter heat treated in a conventional way to obtain the desired hardness of the teeth of the pad; and the inner diameter and outer diameter of the pad are then finish ground. This integral cylindrical pad with a serrated inner surface is then longitudinally sectioned into two or more unitary longitudinal segments, which when reassembled, closely approximate the dimensions of the archetype integral pad. The dimensions will not be identical because of the missing material which is removed during cutting. Sectioning the finish ground integral pad is the final step.
The heat-treated segments of the pad are then assembled in a collet and removably secured therein by any of numerous known fastening means designed to operate under the particular stresses generated within such a collet during use.
From the foregoing it will be evident that the entire integral collet pad, and all longitudinal unitary segments thereof, are of the same material, usually a conventional tool steel; also, that a faulty machining operation, for example, broken serrations during broaching usually results in discarding the entire pad. It will also be apparent that, after a period of use during which the gripping serrations of the unitary segments have become unacceptably worn, all unitary segments of the pad must be replaced. In other words, though only the serrations may be badly worn, the entire pad assembly is usually discarded. In small pad assemblies the cost may not be unreasonable, but in larger pad assemblies, say for stock greater than about 5 cms. equivalent diameter, the cost over an extended period of time is enormous. More importantly, having to unpredictably discard a worn assembly of pads requires that several sets of replacement pads be prudently carried in inventory.
It will also be evident from the foregoing that a conventional, longitudinally segmented pad assembly is fabricated for use with a specific shape of stock and a specific stock diameter. A change of either stock shape or stock diameter necessitates removal and replacement of the pad assembly in its entirety, that is, of all unitary pad segments. Though superficially a jejune task of no great significance, it requires a large inventory of pads, each specifically designed for a particular stock . . . again, a great expense.
Less evident is the fact that currently available automatic screw machines are particularly designed with collets which accept only a relatively narrow range of stock diameters, i.e., each machine is designed for use with pads having inner serrations within a relatively narrow range of diameters. Thus, a machine for relatively large stock may lie idle since a particular smaller stock diameter is not usually run on that machine, simply because it is generally impractical to make conventional longitudinally segmented pads for running 0.5 in. stock in the collet of a machine with a capacity to run 4 in. stock.
The foregoing deficiencies, disadvantages and drawbacks in prior art unitary pad segments are not less readily recognizable by virtue of having become enured to them. Surprisingly, the solution to these problems appears to have consistently escaped those skilled in the art over several score years. The instant invention is directed to an improved collet pad having plural, separate, composite longitudinal segments. The pad dispenses with the necessity of carrying a large inventory of conventional pads, as is now usually done, and simultaneously permits the fabrication of a collet pad which requires no expensive broaching operation.